The present invention relates to electronically tunable ceramic materials which have favorable properties for many applications, including the area of radio frequency (RF) engineering and design. More particularly, the present invention relates to electronically tunable ceramic materials comprising a tunable dielectric phase such as barium strontium titanate (BSTO) in combination with multiple metal oxide phases.
Microwave devices such as electronically scanning antennas, phased array antennas, electronic down tilt antennas, electronically tunable filters, electronically tunable radar and tunable oscillators have been proposed for use, but their high costs have prevented widespread commercial implementation. The existence of low cost tuning technology would revolutionize the industry of microwave components and antennas.
Tuning refers to the ability to change the dielectric constant of a material. This translates in the microwave component field as the ability to use the same component and change the frequency of operation by changing the input voltage or current. It can allow for a single filter to frequency-hop during operation. For antennas, being able to tune using low cost technology would create a new class of phased array antennas, which could be used in common households instead of being limited to large, expensive systems. Phased array antennas can be described as electronically scanning antennas. By using electronically scanning antennas, satellite communications can be available in many situations, because the antennas would allow for tracking of a satellite. This means voice and data communications can be done in a mobile arena. Also, mobile communications would be available at a global level without the need for major infrastructure.
A phased array refers to an antenna configuration composed of a large number of elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. This electronic beam steering concept applies to both the transmitter and the receiver. Phased array antennas are advantageous in comparison to their mechanical counterparts with respect to their speed, accuracy and reliability. The replacement of gimbal mounted mechanical antennas by phased array antennas increases survivability through more rapid and accurate target identification. For example, complex tracking exercises can be performed rapidly and accurately with a phased array antenna system.
Future communications will also require wideband communications using frequency-hopping techniques, so that large amounts of digital data can be transferred over the band. A critical component for these applications is a low cost, fast acting tunable filter. Digital data could be distributed or encoded over a band of frequencies in a sequence determined by control circuitry of the tunable filter. This would allow for several users to transmit and receive over a common range of frequencies.
Technologies for scanning that could possibly be adapted for phase shifter applications are ferrite phase shifters and semiconductor diode phase shifters. Although ferrites are currently the most widely used type of phase shifter materials, they are mostly limited to military applications. The major reason for this is that they are very expensive to manufacture. Secondary reasons include the fact that they are not designed to work in a broadband situation. When the frequency changes, a new set of materials has to be designed and manufactured. They are also very bulky in size and heavy. Furthermore, such phase shifters are difficult or impossible to make in a planar configuration. Conventional ferrites are also driven by high power due to the fact that they activate based on current.
Conventional diode phase shifters are also high cost because they require active electronic circuits to be added to designs in order to compensate for their high loss at microwave frequencies. They do not have good power handling characteristics. When power is put through the diodes, they start to behave in a very non-linear fashion, or they break down. Diodes also require holding power in order to maintain accuracy, as well as power during switching.
Barium titanate is one of the known ceramics tunable at room temperature. Another known tunable ceramic is strontium titanate. However, this material by itself must be super cooled in order to have usable tuning characteristics. Other tunable dielectrics include lead zirconium titanates (PZT), lead lanthanum zirconium titanates (PLZT), PbTiO3, KNbO3, LaTaO3, BaCaZrTiO3, NaNO3 and other ferroelectric perovskites. The problem with these known ferroelectrics is that their losses are very high at room temperature. This makes these materials essentially useless at microwave frequencies. One way to combat this problem is by means of producing a low loss but still tunable composite material with reasonable dielectric constants.
Barium strontium titanate (BSTO) has been used for its high dielectric constant, on the order of 200 to 6,000, and its large change in dielectric constant with applied voltage, on the order of 25 to 75 percent at a field of 2V/micron. Some prior art ferroelectric composite materials which include BSTO are disclosed in U.S. Pat. No. 5,427,988 to Sengupta et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material BSTO-MgOxe2x80x9d; U.S. Pat. No. 5,645,434 to Sengupta et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 6,074,971 to Chiu et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materials with Enhanced Electronic Properties BSTO-Mg Based Compoundxe2x80x94Rare Earth Oxidexe2x80x9d; and U.S. patent application Ser. No. 09/594,837 to Chiu et al. filed Jun. 15, 2000 entitled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric and Metal Silicate Phasesxe2x80x9d, each of which is incorporated herein by reference.
The idea of a voltage tunable dielectric has been proposed for use in antenna applications in a paper entitled xe2x80x9cPlanar Microwave Electro-Optic Phase Shiftersxe2x80x9d, Microwave Journal, Volume 35 (6), June 1992. There is a need for new materials which have desirable electronic properties in order to make this possible. As with most ferroelectrics which are tunable at room temperature, the losses at frequencies above 600 MHz become too high to use cost effectively. Also, high dielectric constants makes it virtually impossible to integrate and match them to RF circuits. This causes increased insertion losses.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
The present invention provides electronically tunable ceramic materials. The materials comprise at least one tunable dielectric phase selected from barium titanate, strontium titanate, barium strontium titanate, lead titanate, lead zirconium titanate, lead lanthanum zirconium titanate, potassium niobium oxide, lanthanum tantalum oxide, barium calcium zirconium titanate, sodium nitrate and other electronically tunable ferroelectrics. A preferred tunable dielectric phase comprises barium strontium titanate of the formula BaxSr1xe2x88x92xTiO3. In the formula BaxSr1xe2x88x92xTiO3, x can be a value from 0 to 1, preferably from about 0.15 to about 0.6.
The present electronically tunable ceramic materials also include multiple metal oxide phases in an amount up to about 80 weight percent of the material. The additional metal oxide phases are used to control the microwave properties, e.g., by reducing the loss tangents and dielectric constants, while maintaining tunability and improving temperature stability. The additional metal oxide phases provide substantially improved characteristics, for example, substantially increased tuning while exhibiting low insertion losses, making the materials useful as tunable dielectrics for microwave applications. The materials also exhibit improved temperature stability, biasing and response time in comparison with conventional materials. Also, the additional metal oxide materials allow the dielectric constant of the tunable dielectric phase, e.g., BSTO, to be adjusted to the desired levels while maintaining tunability. The significant improvements in tunability, stability and adjustibility of dielectric constant without degradation of electric loss enable new RF/microwave applications.
The materials are highly advantageous for microwave component designs because their tuning characteristics allow for the construction of devices having variable properties. Such variable properties allow for new designs, e.g., for filters which can vary the frequency of operation and allow for frequency-hopping of microwave devices, and for antennas to scan and track satellites electronically, as well as many other applications. The electronic properties of the present materials can be adjusted or tailored for a specific microwave application or design. Tunable materials with very low loss insertion are provided. In addition, the materials can be readily used in a wide range of frequencies, e.g., from 100 kHz to 60 GHz. Since the preferred materials require voltage changes in order to switch, very low current and power requirements are achieved. The materials may be provided in many different manufacturable forms such as bulk ceramics, thick films, and thin films.
An aspect of the present invention is to provide an electronically tunable dielectric material including at least one tunable dielectric phase and at least two additional metal oxide phases.
Another aspect of the present invention is to provide a method of forming an electronically tunable dielectric material. The method includes the steps of mixing particles of at least one tunable dielectric material and particles of at least two additional metal oxides, followed by sintering.
These and other aspects of the present invention will be more apparent from the following description.